Monitoring a file until a string is found

Question

I am using tail -f to monitor a log file that is being actively written to. When a certain string is written to the log file, I want to quit the monitoring, and continue with the rest of my script.

Currently I am using:

tail -f logfile.log | grep -m 1 "Server Started"

When the string is found, grep quits as expected, but I need to find a way to make the tail command quit too so that the script can continue.

Answer 1

So after doing some testing, I found a quick 1-line way to make this work. It appears tail -f will quit when grep quits, but there's a catch. It appears to only be triggered if the file is opened and closed. I've accomplished this by appending the empty string to the file when grep finds the match.

tail -f logfile |grep -m 1 "Server Started" | xargs echo "" >> logfile \;

I'm not sure why the open/close of the file triggers tail to realize that the pipe is closed, so I wouldn't rely on this behavior. but it seems to work for now.

Reason it closes, look at the -F flag, versus the -f flag.

Answer 2

This will be a bit tricky since you will have to get into process control and signaling. More kludgey would be a two script solution using PID tracking. Better would be using named pipes like this.

What shell script are you using?

For a quick and dirty, one script solution - I would make a perl script using File:Tail

use File::Tail;
$file=File::Tail->new(name=>$name, maxinterval=>300, adjustafter=>7);
while (defined($line=$file->read)) {
    last if $line =~ /Server started/;
}

So rather than printing inside the while loop, you could filter for the string match and break out of the while loop to let your script continue.

Either of these should involve just a little learning to implement the watching flow control you are seeking.

Answer 3

Alex i think this one will help you lot.

tail -f logfile |grep -m 1 "Server Started" | xargs echo "" >> /dev/null ;

this command will never give an entry on the logfile but will grep silently...

Answer 4

Try to use inotify (inotifywait)

You set up inotifywait for any file change, then check the file with grep, if not found just rerun inotifywait, if found exit the loop... Smth like that